Electrophotographic methods and apparatus to practice such methods have found wide acceptance in the reproduction art. They utilize the property of the photoconductive material to undergo local changes in its electrical resistance upon being exposed to activating radiation.
After being electrically charged and exposed to activating radiation in a pattern determined by an optical image, a photoconductive layer produces a latent electrical charge pattern which corresponds to the optical picture, or image. At the exposed points or areas the conductivity of the photoconductive layer is increased to such an extent that a part or substantially all of the electrical charge can disappear or flow off through the conductive substrate, but in any event the electrical charge disappears or flows off to a greater extent than at the unexposed points or areas where the electrical charge remains substantially unchanged. The electrical charge at the unexposed points or areas which correspond to the stored image can then be made visible with an image powder, a so-called toner, and the resulting toner image, if this should be required, can be transferred to paper or some other carrier.
Organic as well as inorganic substances are used as electrophotographically active substances. Among them, selenium, selenium compounds or alloys with selenium have gained particular importance.
The conductive substrates that have been used can be made of metals, such as, preferably, aluminum, aluminum alloys or steel and brass. Nonconductive carriers which are provided with an electrically conductive coating can also be used as the conductive substrate. For example, a glass carrier covered with an electrically conductive coating of tin dioxide can be used as a conductive substrate.
Electrophotographic recording instruments in which the photoconductive layer is applied to a planar plate as the conductive substrate are not very well suited for rapid, continuous operation. The planar carrier plates are thus often replaced by cylindrical drums as the conductive substrate. These drums, however, have the drawback that, due to their curved surface, the image can be transferred only in strips and the width of these strips is determined by the depth of focus of the reproducing system.
When using drums, it is therefore necessary to move the original that is to be copied or parts of the optical system in synchronism with the rotation of the drum. With high copying speeds, such precision movements are rather difficult as is the transmission of the required quantity of light, because sufficient quantities of light must be able to reach the electrophotographic layer within a very short period of time.
It is possible to increase the operating speed if the photoconductive layer does not have a curved surface but instead has a planar surface because with a planar surface it is possible to simultaneously reproduce wider strips of the original or even the entire original. This is accomplished in conventional electrophotographic recording instruments, for example, by using a flexible coated substrate which is unwound, for example, from a roller and wound onto a second roller or is conducted as an endless belt over two or more rollers. That portion of the belt which is being exposed at the moment is then stretched into a planar plane. For this purpose, belts of aluminum, steel or brass have been used as the conductive substrate because of their good mechanical properties. In addition, plastic belts coated with conductive coatings of aluminum or potassium iodide, such as polyethylene terephthalate polyester films coated with aluminum or potassium iodide, or belts of paper or types of rubber as well as woven or nonwoven, fibrous belts of glass, cotton or silk which have been appropriately made conductive can also be used as the conductive substrate.
The above-mentioned advantages of such flexible carrier belts as the conductive substrate are counteracted by the drawback that it is difficult to obtain sufficient adhesion between the substrates and the photoconductive layer and damage to the photoconductive layer during movement is difficult to prevent. The continuous bending of the photoconductive layer during its travel around the rollers leads to the formation of cracks and breaks, particularly at a high operating speed, because the photoconductive materials are generally relatively hard, brittle and glassy, and even if they have only a very thin layer thickness, the photoconductive layer will finally peel off from the flexible conductive substrates.
It has been believed that these drawbacks of the flexible conductive substrates could be overcome by improving the adhesion between the conductive substrate and photoconductive material. For this purpose, it has been proposed, for example, to roughen the aluminum coating by ion bombardment before the photoconductive material is applied or to apply an intermediate layer with good adhesion with respect to the substrate as well as to the photoconductive material between the flexible substrate belt and the photoconductive layer, which intermediate layer has as little influence as possible on the electrophotographic sensitivity of the system.
Known intermediate layers substantially comprise polymerized organic or silicon organic plastics, such as, for example, nonconductive resin binders with or without additives of phthalocyanine pigments or of substituted sylil isobutyl ethylene diamine. Graphite has also been used as the intermediate layer. The drawback in such intermediate layers is, on the one hand, that there is a noticeable reduction of the electrophotographic sensitivity. On the other hand, the plastic intermediate layers sometimes have layer thicknesses of only 0.1.mu., and this causes added technical difficulties in the production of the recording material, particularly if at the same the layer thickness is supposed to be uniform. Finally, it has been found that, while the use of such intermediate layers might initially improve the adhesion, over a period of time fatigue develops and, as a result of the continuous bending, microcracks are formed which substantially limit the usefulness of the electrophotographic recording material or even make it impossible.